1. Field of the Invention
The invention relates generally to the field of drag reducing devices used with transport vehicles. More particularly the invention relates to specialized shapes for such devices that have greater efficiency in drag reduction.
2. Background Art
It is well understood in the art of physics of fluid flow control over surfaces that an adverse pressure gradient exists when low static and high static pressure regions interact. As a fluid boundary layer flows across a flow control surface there tends to be a blocking boundary formed which can block the flow and even reverse the motion of fluid flow locally. This may in turn separate the fluid with a resulting rise in drag. It is again well understood in the art that generating stream-wise vortices, generally using so-called vortex generators, can create conditions of mixing which may prevent a separation of the flow. There are many types of vortex generators primarily embodied as devices with vanes that protrude above the boundary layer, such as described by Stephens in U.S. Pat. No. 2,800,291. Limitations of the device described in the '291 patent include that the boundary is regulated with increased conditions of drag. Other approaches have been discussed in Kluethe, U.S. Pat. Nos. 3,578,264 and 3,741,285 primarily exhibiting internal boundary vortices when a fluid encounters and is made to flow over a concave surface; in effect forcing the boundary layer between adjacent wings to transfer a vortex into the boundary layer and cause mixing. The results of such devices while of academic merit have inefficiencies due to out of scale issues leading to excess drag.
Wheeler, in U.S. Pat. No. 4,455,045 describes in detail that a strong vortex is formed along the leading edges of delta wings at high angles of attack. In the disclosed device in the '045 patent, submerged channels are molded to have a nominally triangular plan form shape permanently formed onto a flow surface. The channels as described in the '045 patent have sharp upper edges for effective vortex formation, divergent sidewalls essentially normal to the flow-control surface, and floor contours arranged to immediately conduct the stream-wise vortices below the level of the local flow control surface. Basically, the principles described in the '045 patent are that placing a series of submerged channels to be nested together in a properly overlapping manner creates a stream-wise cascade, thereby reducing the drag and at the same time controlling the boundary flow.
There exists a need for improved drag reducing devices for use on long transport vehicles.